It is surprising what you can find hidden even in your local library. At Oxford I managed to find 7 books, two of which were collections of Cecil Sharp, three by Frank Kidson, one by Lucy Broadwood and another by Roy Palmer. Some of these have interesting forwards on what the collectors did with the songs (ie editing) and some with variations in tunes and story of either the same title or different titles.
A recent treasure is Topic's Voice of the People. It is expensive to buy all 20 CDs but maybe the library could be persuaded? This is as close as many of us are going to get to the old recordings without making long journeys.
Another resource is the Bodleian Broadsides and I think folkinfo.org that 'catter Jon Freeman runs.
But yes, the replies you have had so far should get you going. WRT to some labels, there are source singers, song carriers, revivalists and singers, I guess (thinking on the hoof). Source would be part of the tradition and I doubt if many of those are left in the UK because of their age, though notable exceptions are the Copper family of Rottingdean. Song carriers are performers who have collected from source singers (like the Watersons, Shirley Collins), revivalists I guess are the people/leaders who got involved in the fifties/sixties revival and singers, well thats the rest of us.
The EFDSS at Cecil Sharp House in London has a vast archive, but it would be a long, expensive and bewildering trip. Now what would be interesting would be to get an interview with the Coppers, Martin Carthy and Shirley Collins (who has been collecting with Alan Lomax in the US?) - I'm sure all of these people would be happy to help if they had the time. Send them an email explaining what you are trying to do, they are all lovely and approachable people.
Did you get to see Folk Britannia on the telly? Lots of people thought it was flawed, but it was interesting nonetheless.
Finally, there are still gypsies who have a singing tradition. How you would go about this I don't know and what measures you would need to mind, again I don't know.
For people who are performing today, where they get their source from is not necessarily relevant but acknowledging the source is - this gives the listener an idea of where the song came from, the inspiration for doing it and the manner of doing it. It is probably also incumbent upon us to do a bit of research about the history of the song and where it came from - but hey, hours in the day and all that.
Good luck, an exciting time lies ahead. Do you play/perform yourself?